1. Field of the Invention
This invention relates to low viscosity, high solids content polyesterdiols, their use in high solids content, curable resin compositions and curable coating compositions containing these polyesterdiols which are essentially free of volatile organic compounds.
2. Description of Related Art
Polyester diols having a relatively low viscosity and a relatively high content of non-volatile material (NVM) are known in the art. These materials are useful as components in curable formulations such as coatings, paints or adhesives and as precursors in the preparation of polyurethanes.
These polyester diols are generally prepared by reacting C.sub.4 to C.sub.20 aliphatic, cycloaliphatic, or aromatic dibasic acids or acid derivatives (or mixtures thereof) under esterification conditions with a molar excess of one or a mixture of aliphatic polyols, to produce polyester diols having the general structure: EQU HO[R'OOC--R"--COO].sub.m R'--OH
where R' is the organic residue of an aliphatic polyol and R" is the organic residue of a dibasic acid or acid derivative. Polyester diols, also referred to as polyester di(poly)ols are telechelic oligomers which are especially useful as binder components in curable resin systems, as stated above. Because of concerns in the paint and coating industry with respect to reduction of solvent (volatile organic compound or VOC) emissions into the atmosphere during baking of the resin composition, there is a considerable interest in providing reactive polyesterdiol components which have both very low viscosity and very low VOC content. The lower the polyesterdiol viscosity, the less solvent will be required to be added to the resin composition in order to produce a coating or paint which is readily sprayable on substrates. However, even in the case of low molecular weight polyesterdiols, reduced solvent content results in a coating composition too viscous to be applied by spray at low temperatures. For example, U.S. Pat. No. 4,243,705 teaches spray application of such coatings at temperature up to about 250.degree. F.
U.S. Pat. No. 3,994,851 teaches high solids coating compositions prepared from a mixture of an ester-containing polyol having a molecular weight of less than about 850 and an amine-aldehyde crosslinking agent. Although the reference describes the preparation of ester-containing polyols having a viscosity of as little as 1.4 to 12.9 poises (about 140 to 1290 cps), these viscosities appear to be based on the ester-containing polyol "as synthesized", which would contain a considerable amount of unreacted polyol diluent. The reference does not disclose that unreacted polyol is removed. Thus, where the reference describes high solids coatings which may have a solids content of up to 100%, consideration of quantities of unreacted polyol present in the ester-containing polyol is evidently not taken into account.
In many cases, the term NVM means that the polyester does not contain any added solvent and does not mean 100% NVM as defined by ASTM D2369-90. Sometimes it is referred as "Theoretical" or "Calculated" NVM. For example, Cargill (now McWhorter) commercial 100% NVM polyesters 57-5763, 57-5879 and 57-5880 have NVM measured according ASTM D2369-90 of about 91%, 72% and 74% respectively and, therefore, contain significant VOC which is primarily unreacted diol. U.S. Pat. No. 3,852,375 discloses the preparation of curable varnishes and enamels comprising a mixture of an ester diol having a molecular weight of from about 200-600 derived from aliphatic diols and a mixture of aliphatic and aromatic or cycloaliphatic diacids, and an aminoplast crosslinking resin. In this patent, the authors describe measures to keep an excess of diol from the start to the end of the synthesis process without any attempt to remove excess of diol.
U.S. Pat. No. 4,104,240 describes the preparation of low molecular weight and narrow molecular weight distribution polyester diols. The authors teach reacting excess diol (2-4 moles) per 1 mole of diacid in presence of catalyst, followed by vacuum distillation at 90.degree. C. This low temperature for distillation was chosen to achieve essentially non-reacting conditions and avoid transesterification during distillation. The authors do not provide data relating to NVM or viscosities of these products. It should be also recognized that 90.degree. C. is not a commercially viable temperature to achieve quantitative distillation of high boiling temperature diols from polyesterdiol due to excessively long processing times.
U.S. Pat. No. 4,104,240 also discloses curable pigmented coating compositions containing an amino or polyisocyanate curing agent and a mixture of two different ester diols, one of which is the esterification product of a divalent alcohol and a cycloaliphatic and/or aromatic dicarboxylic acid (molecular weight 254-2000) and the other of which is the esterification product of a divalent alcohol and an aliphatic dicarboxylic acid (molecular weight 178-2000). The pigmented, cured compositions are said to have improved flexibility and hardness.
Similar processes are disclosed in U.S. Pat. No. 4,888,441 and related U.S. Pat. No. 4,922,002. In these disclosures, a dibasic ester of a dicarboxylic acid is reacted with an excess of a polyhydric alcohol and in the presence of an esterification catalyst to produce polyester polyols, followed by stripping excess unreacted polyhydric alcohol using a wiped film evaporator at temperatures of 150-225.degree. C. and 0.05-200 torr, and cooling the resultant product to prevent further reaction. The use of a wiped film evaporator reduces high temperature residence time during distillation and therefore substantially reduces transesterification effect on the structure and viscosity of the polyesterdiol. The products are characterized as containing less than about 4% by weight of polyhydric alcohol and have viscosities of above 3700 cps as made in the examples.
A method equivalent to the process disclosed in U.S. Pat. No. 4,104,240 is also disclosed and claimed in U.S. Pat. No. 4,540,771. However, despite listing many commercially available monomers for polyester diols, the authors demonstrate their invention only using polyesterdiols of cycloaliphatic or aromatic diacids which, by their chemical structure, provide polyesterdiols with significant resistance to transesterification. Polyesterdiols having a solids content in excess of 95% are disclosed in some of the examples, but all of these materials have viscosities in excess of 60 poises (6000 cps).
It is known in the literature (P. Flory, JACS, V62, p. 1057 (1940), that polyesters based on aliphatic dicarboxylic acids are prone to transesterification even in the absence of a catalyst used in their synthesis, and at a temperature as low as 109.degree. C. Therefore, to prevent the negative effect of transesterification in the preparation and stripping of very low molecular weight polyesters, researchers resorted to very low distillation temperature (U.S. Pat. No. 4,104,240), very short distillation time (U.S. Pat. No. 4,922,002) or simply avoided the use of aliphatic diacids (U.S. Pat. No. 4,540,771).
Transesterification of the polyesterdiol during stripping can lead to a product having an increased molecular weight, a broader molecular weight distribution and an increased viscosity of the stripped polyesterdiol. In addition, transesterification is accompanied by the development of monomeric diol as a transesterification by-product which is counterproductive to achieving a product having a high NVM content.
Thus, the goal to achieve aliphatic polyesterdiols of low viscosity and having a very low content of residual diol using post-synthesis stripping techniques is counteracted by the problem of viscosity and molecular weight increase of the polyesterdiol which takes place as a consequence of the stripping step. Viscosities of polyesterdiols depend on their monomer compositions and, even at the same molecular weight and molecular weight distribution, viscosities can be significantly different.
Therefore, an important goal of the invention is to provide a definition of the optimal molecular weight distribution providing optimal viscosity for any particular polyesterdiol comprising aliphatic diacid moieties, technique to measure their molecular weight and molecular weight distribution, industrially viable technique to produce such polyesterdiols and the polyesterdiols themselves. High purity polyesterdiols produced according to the process of the present invention are among the goals of the present invention.
Accordingly, the present invention provides polyesterdiol materials having both a very low viscosity below 3500 cps and a very high content of non-volatile material in excess of 96 wt % and up to about 100 wt %.
The invention also provides processes for the production of aliphatic polyester diols wherein transesterification reactions encountered during heat stripping of unreacted diol are markedly reduced.
The invention provides crosslinkable compositions providing improved properties and lower VOC comprising the polyesterdiols of the present invention.
The major function of solvents in coating composition is to reduce viscosity to the level necessary for applicability by techniques developed for liquid coating compositions, especially for spray.
U.S. Pat. No. 4,243,705 described significant increase in temperature of coating compositions (up to 250.degree. F.)--hot spray to reduce viscosity without further increase of solvent content in coating compositions. This method still has some limitations--high temperatures significantly increase fire hazard; at high temperatures curable coating compositions become unstable due to the crosslinking process.
Another interesting approach is substitution of part of the VOC generating solvent with VOC free water (Olson, M. R., Larson J. M., Jones, F. N., J. Coat Technol. 55(699) p. 45, 1983; Blank, W. J., J. Coat. Technol. 60(764), p. 43, 1988; M. R. Olson, U.S. Pat. No. 4,616,054 Beresniewicz, A. U.S. Pat. No. 4,294,738). In this approach many water miscible solvents, especially glycols and their derivatives significantly increase compatibility of poly(oligo)meric components with water allowing the system to imbibe more water and correspondingly to achieve more reduction in viscosity. However, the water-miscible glycols and their derivatives are VOC-generating solvents and reduction in viscosity is limited by solubility of water in the poly(oligo)mer-glycol (glycol derivative) mixture and increases with the increase of the level of the VOC-generating components.
Nevertheless, despite numerous efforts, industry still does not have the solvent-free curable compositions with low viscosity applicable by techniques developed for liquid coating compositions.
The invention provides liquid, curable, polyesterdiol-containing coating compositions which are essentially free of VOC-generating solvents, including unreacted diols, and which can be applied to substrates at low temperature using conventional application methods.